Elements of The Transformers as we know them existed years prior to the creation of the brand.

Takara had two transforming robot toylines, Diaclone and Microchange (a sub-line of Microman). Nither line were selling well, even in the US.

Hasbro said, give us the toys and we'll market them right. So Hasbro took a selection of random Diaclone and Microchange toys and then turned to Marvel/Sunbow to come up with a story and characters based on the toys.

Marvel/Sunbow created the Transformers, Autobots, Decepticons, Cybertron and everything storywise that makes them the Transformers. The Marvel comic and the Sunbow animated series were both released in 1984.

Hasbro later bought more molds to bolster their toyline from Toybox, Toyco, Bandai and Takatoku. Marvel/Sunbow then worked those toys into characters for the fiction.

So the toys the Transformers were based on existed for several years between various companies and toylines, but weren't brought together as the "Transformers" until 1984.